Add Strangler Fig migration pattern

[JerrettDavis]

Goal

Add Strangler Fig as a first-class PatternKit pattern for incrementally routing traffic between legacy and replacement capabilities during modernization.

Required scope

• Fluent runtime API for named migration routes, predicates, and legacy/new handlers.
• Source-generated factory path with diagnostics for invalid route declarations.
• TinyBDD runtime, generator, and real-world example coverage.
• Importable IServiceCollection and ASP.NET Core routing example.
• User guide, generator guide, example docs, catalog coverage, and production readiness entries.
• CI must land green on main.